Uncountable Nouns

Some nouns are not countable and are called uncountable nouns or mass nouns.  

For example, the word clutter is a mass noun.

That garage is full of clutter.

This sentence makes grammatical sense.

However, the following example does not.

That garage is full of clutters.

Mass nouns cannot take plural forms.

Substances, liquids, and powders are entities that are often signified by mass nouns such as wood, sand, water, and flour.

Other examples would be milk, air, furniture, freedom, rice, and intelligence.